1. Field of the Invention
This invention relates to video processors for energy reception systems and particularly to detection arrangements that provide a constant false alarm rate for substantially any probability distribution of the received clutter and noise energy.
2. Description of the Prior Art
Conventionally, mean level detectors have been utilized to implement an adaptive threshold which samples the video amplitude in contiguous range and/or azimuth bins and changes the detection threshold in response to changes in the mean or average value of the clutter. A mean level detector compensates only for changes in the average value which is the first moment of the probability density function of the clutter. Thus, the mean level detector is only effective against clutter with a single parameter probability density function such as the Rayleigh density function. The Rayleigh probability density function is given by ##EQU1## WHERE X IS THE VOLTAGE AND .alpha. IS THE STANDARD DEVIATION OF THE Gaussian noise before envelope detection, and the mean or expected value is given by m = .sqroot..pi./2 with the standard deviation ##EQU2## Thus, both the mean and the standard deviation are functions only of .alpha., and sigma .sigma. = .52272m so that a Rayleigh distribution and all of its moments are completely defined by a single parameter. The mean level detector by estimation of the mean value can thus compensate for any changes in Rayleigh distribution clutter. The probability of a false alarm is ##EQU3## so that the false alarm rate can be completely controlled by setting the threshold t as a specified function of m. As a result, the probability of a false alarm rate remains constant with changes in the mean value because the threshold is uniquely determined by the mean. However, for distributions with more than one parameter, the mean level detector does not provide a constant false alarm rate and if the sigma is doubled, for example, the false alarm rate would increase by a large amount. A detecting system that utilizes a plurality of moments to establish the detection threshold would be a substantial advantage to the art in handling various clutter distributions. Some prior art systems have utilized first and second thresholds with the mean deviate estimate utilized to estimate the standard deviation of the reference cells such as in an article, "Modified Generalized Sign Test Processor For 2-D Radar" by G. V. Trunk et al, published in the IEEE Transactions On Aerospace And Electronic Systems, Vol. AES-10, No. 5., Sept. 1974. This prior art technique utilizes the absolute value of the deviations rather than the second moments and it has been found to require a substantially larger number of samples in order to provide the accuracy that would result from the use of the first and second moments.